This invention relates to a system and method for dewatering or separating particles from a liquid or colloidal suspension which uses an electrofilter with an improved electrode assembly for attracting the particles.
Nearly all phase boundaries exhibit an electric charge. This is due to the asymmetric forces acting on the interface. Such surface charges impart properties to particles which are different from the bulk material. For instance, a dispersion of very fine particles in water will not settle because the downward force due to gravity is less than the upward repulsive forces due to the surface charge of the particles.
If solid particles are dispersed in a liquid so that they acquire a surface charge, and an external electric field is applied, the particles will be attracted towards the electrode of opposite charge and repelled by the electrode of like charge. If the particle is free to move, the motion imparted by the electrical gradient is called "Electrophoresis". If the particle is restrained, then the liquid will move relative to the particle so that relative to the liquid, the particle appears to be moving. This phenomenon is called "Electro-osmosis".
The rate of movement of the particles and/or liquid is proportional to the intensity of the applied electric field and the magnitude of charge at the particle/liquid interface. That surface charge can be modified by changing the composition of the solid or liquid including such properties as pH, conductivity, and the addition of other materials.
An electrochemical cell is formed by the juxtaposition of electronic and electrolytic conductors so that as electricity is passed from the electronic conductor, such as a metal, to the electrolytic conductor, such as an acid, base, or salt solution, a chemical reaction occurs at their interface. The electronic-electrolytic interface at which oxidation occurs is known as the anode or positive electrode and that at which reduction occurs is known as the cathode or negative electrode.
When charged particles are placed between the anode and cathode of the electrochemical cell, the particles will tend to migrate towards an electrode as a function of their charge. For example, kaolin clay particles are negatively charged so they migrate towards the anode. Water molecules tend to move towards the cathode by electro-osmosis. Accordingly, a kaolin clay suspension can be "dewatered" by placing the clay suspension within an electrochemical cell and applying a direct current. The clay particles settle or deposit on the anode surface and on each other, displacing water or liquid molecules to form a more dense cake layer or slurry with a higher solids content than that of the liquid or aqueous suspension. The anode is removed from the cell and the dense layer of clay particles is recovered by removing the layer from the anode. The water is collected or removed at the cathode.
Since the rate of migration of the particles is a function of current density, it would seem desirable to apply as much current as is possible. However, since the generation of acid (H.sup.+) and base (H.sup.-) by hydrolysis of water molecules and the amount of oxidation at the anode is propotional to the amount of current, the applied current is limited to reduce these reactions which would otherwise shorten the electrode life.
It is, therefore, desirable to provide electrodes which are highly conductive, efficient and resistant to acid and oxidation. Costs should also be as low as possible since the electrode must be replaced if it corrodes. Much work has been directed towards electrode assemblies which will provide maximum performance at minimum costs. Alteration of other aspects of the system has also been explored. For example, the electrolyte may be continuously replenished so that the reaction products are removed or neutralized.
An example of a dewatering system utilizing the method of placing a suspension between electrodes and applying an electrical current is described by U.S. Pat. No. 4,367,132 to Bell et al. In this method, dewatering of chemically precipitated sludge is achieved by passing direct electric current through the sludge between a pair of submerged perforated electrodes. As a result of this treatment, the liquid phase of the sludge flows by electroosmosis towards the cathode where it is collected after passing through the perforated cathode. Additional liquid is accumulated at the perforated anode where it diffuses as the sludge solids accumulate on the anode. Since the anode may be consumed in the process, the perforated electrodes used in this method are relatively simple metal sheets preferably constructed of low cost materials such as iron, aluminum or graphite. The electrodes are optionally covered with a liquid porous, non-conductive membrane made of a material such as polypropylene or rayon fabric to prevent the sludge material from clogging the electrode. (See column 5, lines 5-27 of the '132 patent.)
In an important commercial application of electrofiltration, as applied to the separation of kaolin clay particles, it has been found that a titanium electrode, in the form of a sheet to which an expensive protective coating of an acid and oxidation resistant conductive metal catalyst is applied, is the only electrode that is practical. This electrode is very expensive because it is formed of titanium and the protective coating of acid and oxidation resistant metal catalyst costs about $100 per square foot. The function of the coating is to protect the titanium backbone from the corrosive environment of the electrofilter. It does this by catalyzing an electrode reaction such as the hydrolysis of water at a lower potential than that which would oxidize the base metal. In practice, however, the protective coating material does slowly erode, or deactivate, revealing the titanium backbone which then erodes. The replacement cost of these electrodes is substantial. It is therefore desirable to eliminate the need for a separate electrode element within the shell which would otherwise have to be formed of a material that is acid and oxidation resistant, such as the coated titanium.
Various electrode assemblies for use in electrolysis cells, which are more resistant to corrosion, are described in U.S. Pat. Nos. 4,191,618 to Coker et al., 4,323,435 to Carlin and 4,360,416 to Davidson et al. These electrodes are constructed by bonding metal composites to the surface of ion-exchange membranes and are known as solid polymer electrolyte assemblies. The metal composite generally contains at least a catalytic material such as a platinum group metal oxide and optionally a manganese or valve metal oxide to improve stability to oxidation and/or a hydrophobic compound such as Teflon. The ion exchange membrane transports cations generated during electrolysis at the anode so that the cations may move towards the cathode, while the membrane remains substantially impermeable to the flow of liquid. Back migration of the caustic (OH.sup.-) to the anode is prevented since anions are unable to pass through this type of membrane. These electrode assemblies are more resistant to corrosion, but have been limited to use within electroylysis cells where the anode and cathode are almost adjacent and are separated only by the ion selective membrane.
Variables effecting the performance of an electrode in an electrochemical cell include the surface area of the electrode, the presence of and nature of a catalyst metal and/or conductive metal, contaminants in the reactants, and the nature of the reactions taking place in the cell. Consequently, it is difficult to predict the applicability of an electrode useful in one electrochemical cell system to a different system even though one type of electrode may produce advantageous results in one type of electrochemical cell system. It does not always follow that such an improvement will be realized when the same electrode is utilized in a different electrochemical cell system. It is always desirable not only to improve the stability of the electrode and other elements of the electrochemical cells, but to improve the efficiency of the electrochemical cells in the processes carried out therein.
Accordingly, it is an object of the present invention to provide an electrofilter with an electrode assembly which is minimally affected by adverse reaction products such as acid and caustic, and therefore has a long life.
Another object of the present invention is to provide an electrofilter with an electrode assembly that is inexpensive to manufacture and operate.
A further object of the present invention is to provide an efficient and inexpensive method for the dewatering or separation of particles from a liquid or colloidal suspension by employing an electrofilter with an improved electrode assembly.